1. Field of the Invention
The present invention relates to a method of three-dimensional (3D) reconstruction of arborescence by labeling. The invention is primarily intended to be employed in the medical field in which the arborescences studied are angiographic arborescences. By subsequent processing of information on the reconstructed object, three-dimensional reconstruction makes it possible to present the object in any desired mode: transverse cross-sections, oblique cross-sections or even 3D display. It should be added that 3D display of 3D objects is already known The invention is essentially concerned with acquisition of geometrical data which are representative of a 3D arborescence, these data being subsequently employed in methods of visualization for displaying the arborescence. The distinctive feature of the method in accordance with the invention is that it permits reconstruction of arborescences from two-dimensional digital images in projection of the object to be reconstructed
The field of application of the invention is in particular the study of the vascular system (arterial and venous system) of any region of the human body which has a treelike structure (heart, brain, femoral artery, carotid artery, etc.). The mode of acquisition of images in projection is independent of the method. Although the invention is described in a radiology application, this latter is transposable to the case in which the images by projection are obtained by NMR, by ultrasonic insonification, and so on. Digital or analog radiology by x-rays (angiographic technique) makes it possible at the present time to obtain images which are well-suited to the practical application of the invention. The method in accordance with the invention is also applicable to any 3D filar structure other than medical.
2. Discussion of the Background
Current angiographic reconstruction techniques consist partly of techniques derived from tomodensitometric experimentation involving the use of scanners. However, the corresponding acquisitions are complicated, firstly by the need to remove from acquired images the contributions of all that does not represent the angiographic system, secondly by the fact that the flow of blood within the vessels is a phenomenon which is variable with time (and therefore calls for synchronization) and finally by the fact that the acquisition must be a three-dimensional acquisition. In order to eliminate contributions to the images by elements which are foreign to the angiographic system, it is a known practice to utilize injections of products which enhance the contrast within the capillaries. It will be borne in mind, however, that these injections cannot be repeated as often as may be desired without traumatizing the patient. The synchronization phenomenon may have the effect of increasing the duration of acquisitions. At the same time, this technique is contrary to the precautions which are necessary in order to avoid over-frequent injection of the contrast-enhancing product into a patient's blood vessels. Finally, when making use of scanner methods, three-dimensional reconstruction calls for repetition of these experiments. One of the solutions to this problem would consist in employing multi-row multidetectors in the scanners. However, this technique is essentially related to the systematic use of so-called conic projections since the x-ray source remains a point source. The algorithms of reconstruction of cross-sectional images from conic projections do not subsequently make it possible to achieve the requisite precision for permitting reconstructions. In order to overcome this disadvantage, a scanner has been designed to acquire the images of four cross-sections at the same time. The complexity of this machine is clearly multiplied by the number of simultaneous cross-sections which it is desired to acquire.
Scanner acquisition is nevertheless subject to a disadvantage: it takes place in the course of time and, in particular when it is sought to represent moving organs such as the heart, it can finally provide only blurred images of the part of the body to be displayed. In order to avoid problems of synchronization (and resultant multiple injections of contrast-enhancing products), there has been designed an apparatus equipped with fourteen x-ray generators each associated with a camera. Each of the fourteen pairs is then capable of producing an image in projection of the structure to be reconstructed. This system surrounds a volume of the body from which a numerical volume is produced. A numerical volume is a collection of data relating to a measured property and arranged virtually in a volume at 3D addresses corresponding to the locations of the object from which the information is obtained. Interesting results have thus been obtained for the reconstruction of coronaries. This system makes it possible in addition to display all the structures which exhibit an attenuation to x-rays. However, the disadvantages of this system are twofold. In the first place, they are of a technical order: the cost of the equipment is incompatible with industrial diffusion. Furthermore, the definition of the images is not sufficient for detection of fine structures. Should it be desired to obtain a resolution which is adapted to these fine structures, the number of data to be acquired and to be processed over a period of time compatible with medical use imposes an appreciable increase in the power of the machines. In addition, the problem is of a theoretical order: the x-ray beam employed is a divergent beam. The "parallel cross-section" approximation employed for the reconstruction is therefore rough. Utilization of conical geometry in the reconstruction algorithms precludes any possibility of resolving the problem into a superposition of two-dimensional reconstructions.
Another technique has been employed. This technique consists of an algorithmic approach by searching for homologous points. This method consists in determining homologous points on images in projection. Homologous points are points of images of each projection which are associated and which correspond to one and the same point of the 3D space of the arborescence to be reconstructed. The algorithmic method makes it possible to calculate the 3D coordinates of the point of the object from a knowledge of the images acquired and of the geometry of the acquisition system. The methodology employed in this case is as follows. A characteristic point is sought with a first algorithm on a first image in projection. This characteristic point is located on the path of a particular x-ray. The path of the x-ray is known as a "3D straight line". The method consists in projecting the 3D straight line on the second image in projection by making use of the second projection orientation. The homologous point of the characteristic point chosen must be sought in the second image in projection: it must be located on the projected 3D straight line known as an epipolar line.
The most reliable characteristic points for the vascular trees are the points of bifurcation. In fact, the images in projection have patterns which are essentially of two types. In a first type, Y-patterns represent a bifurcation: a main blood vessel is divided into two secondary vessels. In a second type, the patterns represent X intersections: in the majority of cases, these intersections do not correspond to any particular structure within the body. In fact, they are only the result of the projection, on a plane, of two independent segments, only the images of which intersect each other.
Automatic localization of the characteristic points calls for effective segmentation of angiographies. The accuracy which is necessary in the determination of a homologous point in the stereoscopic condition in order to ensure that the estimation of the X, Y, Z coordinates of the corresponding points in the object is acceptable, is less than one pixel. This constraint can be made more flexible if it is possible to increase the angle between shots, between the directions of projection. This can be obtained by making use of views having projection orientations which are spaced at an angle close to 90.degree..
In a particular version of the method, three images in projection are acquired at orientations included in an angle of 90.degree.. This accordingly solves the principal difficulty of conventional methods which is that of reconciling two opposite constraints in order to obtain sufficient accuracy of the 3D precision of an element of the object. With orientation spacings of the order of 90.degree. between projections, the result thereby achieved is to improve the accuracy of calculation of coordinates of the point of intersection of the straight lines corresponding to the homologous point. On the other hand, by choosing views having small angles of projection with respect to each other, the problem of correspondences between images can be solved. In the final analysis, relinquishment of scanner techniques can lead to simplification of equipments for the reconstruction of arborescence. However, in the last method mentioned above, as also presented in a French Patent Application No. 87 16156 filed on Nov. 23, 1987, it was still necessary to provide three x-ray tube--camera equipments. With these three equipments, one acquires the three images, the projection orientations of which make an angle between them respectively of 90.degree. and of a few tens of degrees.